Wifi can be horrible slow with lots of packet loss on my PB too, but after configuring my router to use channel 1 it's been fine. When I had that in "auto" mode, it was pure luck when it worked fine and when not. So I think it's a bit sensitive about the channel and I'd try to use channels 1 or 6 which are the best wifi channels in theory. 11 could be too, but it's been said that at least for certain age Apple hardware you shouldn't use channels over 10 or so.. so I'd try 1 or 6 when testing if it's about this.